In a conventional photomultiplier tube, electrons emitted from a photocathode provided on an end of a vacuum vessel are multiplied by dynodes and detected by anodes, and a stem constituting the other end of the vacuum vessel is made of large, tapered hermetic glass, and a metallic tip tube is fusion bonded to the center of the stem and protrudes downward as a metallic air discharging tube (for example, refer to patent document 1).
Further, another known photomultiplier tube has a configuration that a dish-shaped stem metallic plate is disposed such that the stem metallic plate may surround the outer surface of the stem constituting the other end of the vacuum vessel, and that an air discharging tube is hermetically engaged with and fixed to the dish-shaped stem metallic plate (for example, refer to patent document 2).
Patent document 1: Japanese Patent Application Publication No. H5-290793 (page 4, FIG. 7)
Patent document 2: Japanese Patent Application Publication No. 2005-11592 (page 3, FIG. 1)